1. Field of the Invention
The present invention relates to a display device and method of fabricating a display device and, more particularly, to a liquid crystal display device and a method of fabricating a liquid crystal display device.
2. Description of the Related Art
As mobile electronic devices, such as a mobile phone, personal digital assistants (PDAs) and notebook computers, are developed, flat panel display devices having light weight and thin profiles are required. Various types of flat panel display devices are being developed including liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and vacuum fluorescent display (VFD) devices. Among these various devices, the LCD devices are appealing because of their mass production techniques, ease of driving, and implementation of high picture quality.
In the liquid crystal display device, there are various display modes according to arrangement of liquid crystal molecules of a liquid crystal layer. A Twisted Nematic (TN) mode is commonly used because of its simple display of black and white images, fast response time, and low driving voltage. In the TN-mode liquid crystal display device, liquid crystal molecules that are initially aligned along a horizontal direction to the substrate are subsequently aligned almost vertically to the substrate when a voltage is applied to the liquid crystal layer. Accordingly, viewing angle becomes narrow due to a refractive anisotropy of the liquid crystal molecules when the voltage is applied.
To solve the viewing angle problem, there have been proposed LCD devices with various display modes having wide viewing angle characteristics. Of the LCD devices, an in-plane switching (IPS) mode liquid crystal display device has been adopted in which at least a pair of electrodes are arranged in parallel within a pixel region to form a horizontal electric field substantially parallel to the surface of a substrate, thereby aligning liquid crystal molecules within a single plane.
FIG. 1A is a plan view of an in-plane switching (IPS) mode liquid crystal display device according to the related art, and FIG. 1B is a cross sectional view along I-I′ of FIG. 1A according to the related art. In FIGS. 1A and 1B, a pixel region of a liquid crystal display panel 1 is defined by horizontal and vertical arrangements of a gate line 3 and a data line 4, and a thin film transistor 10 is formed at an intersection of the gate line 3 and the data line 4 of the pixel region. The thin film transistor 10 includes a gate electrode 11 formed on a first substrate 20, a gate insulating layer 22 formed over the first substrate 20, a semiconductor layer 12 formed on the gate insulating layer 22, a source electrode 13 and a drain electrode 14 formed on the semiconductor layer 12, and a passivation layer 24 formed over the first substrate 22. In addition, a plurality of common electrodes 5 and a plurality of pixel electrodes 7 are arranged substantially parallel to the data line 4 within the pixel region, and a common line 16 and a pixel electrode line 18 are connected to the common electrode 5 and the data line 7 at a middle portion of the pixel region. The common electrode 5 is formed on the first substrate 20 and the pixel electrode 7 is formed on the gate insulating layer 22. A black matrix 32 and a color filter layer 34 are formed on the second substrate 30, and a liquid crystal layer is disposed between the first and second substrates 20 and 30, thereby completing the IPS mode liquid crystal display panel 1.
In the IPS-mode liquid crystal display device, the liquid crystal molecules are aligned substantially parallel to the common electrode 5 and the pixel electrode 7. When a signal is supplied to the pixel electrode 7 as the thin film transistor 10 is enabled, a horizontal electric field is generated between the common electrode 5 and the pixel electrode 7, which is substantially parallel to the liquid crystal display panel 1. Accordingly, the liquid crystal molecules are rotated within the same plane along the horizontal electric field, so that a gray level due to the refractive anisotropy of the liquid crystal molecules can be prevented.
However, in the IPS-mode liquid crystal display device, if a vertical electric field is generated along a vertical direction to the liquid crystal display panel 1 (that is, in the direction from the first substrate 20 to the second substrate 30), the vertical electric field adversely affects the horizontal electric field of the liquid crystal layer 40. Accordingly, the horizontal electric field formed on the liquid crystal layer 40 is not formed to be completely parallel to the liquid crystal display panel 1. Thus, the liquid crystal molecules of the liquid crystal layer 40 are not rotated within the same plane and create a defective liquid crystal display panel.
One common explanation for formation of the vertical field concludes that the vertical electric field is generated by static electricity formed at a rear surface, which is exposed without being in contact with the liquid crystal layer, of the second substrate 30. The static electricity is generated when the rear surface of the second substrate 30 contacts a human hand, or the like, after completion of the liquid crystal display panel 1. Accordingly, in the IPS-mode liquid crystal display device, a static electric removing conductive layer 36 of a transparent conductive material, such as Indium Tin Oxide (ITO), is formed at the rear surface of the second substrate 30 where the color filter layer is not formed to prevent accumulation of the static electricity. Although not shown in the drawings, the conductive layer 36 can dissipate the static electricity generated at the second substrate 30 through ground.
The conductive layer 36 is formed prior to a color filter formation process. Accordingly, as the second substrate with the conductive layer 36 formed thereon is conveyed to a color filter line, a color filter is formed.
FIG. 2 is a flow chart of a method for fabricating a color filter substrate of an IPS mode liquid crystal display device according to the related art. In FIG. 2, Step S101 includes forming a transparent conductive layer, such as ITO, on a rear surface of a second substrate by evaporation or sputtering processes.
In Step S102, the conductive layer 36 is annealed for about 130 to 140 seconds at a temperature of about 220° to 240° C. Accordingly, since the transparent conductive layer formed at the rear surface of the second substrate is amorphous, the transparent conductive layer has low transmittance and is not suitable for a transmission-type LCD device. Thus, the transparent conductive layer should be crystallized using an annealing process to improve the low transmittance.
In Step S103, the second substrate and the transparent conductive layer are crystallized through the annealing process and conveyed to the color filter layer formation system.
In Step S104, the rear surface of the second substrate, where the conductive layer is formed, is reversed to face upward upon conveyance to the color filter layer formation system.
In Step S105, the second substrate is loaded into a conveying unit, such as a cassette, to the color filter formation system for proceeding with formation of the color filter layer. Accordingly, since the rear surface of the second substrate faces upward, the front surface of the second substrate where a color filter layer is to be formed is loaded to be in contact with a guide bar of the conveying unit. However, foreign material is generated at the front surface of the second substrate by contacting the guide bar of the conveying unit. Accordingly, the foreign material hinders formation of a uniform color filter layer and damages rubbing fabric used during the subsequent rubbing process of an alignment layer coated on the second substrate.
The second substrate is maintained at a temperature of about 100° C. while being loaded into the conveying unit. Then, the second substrate is annealed at a temperature of 220° C. to 240° C.